The invention relates to a drying balance with a pan which rests on a weighing system and a heat source for heating and drying a sample on the pan. The invention further relates to a method for operating this drying balance.
Conventional drying balances are disclosed for example, in German Patents DE 36 15 660 C2 and DE 199 56 372 C2.
Drying balances of this type are relatively fast compared to those using a drying cabinet method. However, it still requires several minutes to determine the moisture content of the sample. In industrial production the moisture content of a material must be determined within no more than a few seconds so as not to slow down the production process. For that purpose, drying balances of this kind are not usable. Instead, indirect measurement methods are frequently used. One example for such an indirect measurement method is the method of measuring optical absorption.
Calibration curves of indirect moisture measuring devices are typically recorded point for point by measuring different samples with different moisture contents. At the same time the indirect moisture measuring device and the known drying balance are separately used to determine the calibration curve on the basis of the value pairs thus obtained. This method requires the production of a plurality of samples and is therefore complicated and time-consuming.
German Utility Model DE 88 02 378 U1 discloses a drying balance with built-in temperature sensor for monitoring and controlling the output of the heat source. A temperature calibration disk with an additional temperature sensor can be connected to the drying balance. By applying a thin layer of the sample material, the temperature calibration disk is given the absorption coefficients of the sample material. In a calibration cycle, the temperature sensor built into the drying balance can be calibrated relative to the additional temperature sensor in the temperature calibration disk. In a subsequent measurement cycle, the calibration curve is used to control the heat source. The actual samples are used in such a way that the specified drying temperature of the sample is met. DE 88 02 378 U1 further provides for the use of an easy-to-handle temperature disk as a secondary standard. The absorption coefficient of the sample material is simulated by strongly and weakly absorbing surfaces in the correct area ratio, wherein a one-time measurement must be used to determine the correct area ratio relative to the temperature calibration disk with the sample substance.